Such a roller is known from a company publication of the Japanese firm Tokuden. Provision is made inside the roller, coaxially to its axis, for a coil arrangement which heats the roller body inductively from inside. Disposed radially outside of the coil arrangement in the roller body are bore holes which are parallel to the axis and are partially filled with water, because water has the greatest evaporation heat and, thus, the greatest temperature-adjustment effect during condensation. The bore holes are each closed individually at the ends in a pressure-resistant manner. If a temperature drop occurs at a bore-hole location during operation, the vapor in the free space of the hollow chamber not filled by water condenses there and produces a condensation temperature-rise which immediately brings the location in question to the temperature of the ambient environment. Thus, there is an automatic equalization of temperature over the roller-body surface.
The elongated channels can be produced in the roller body at relatively low cost using known means, leaving the stability of the roller body essentially intact. The expression "elongated channel" is used to imply that the cross-section is not important, and that the channels have a length amounting to a multiple, e.g. 20 to 150 times, the cross-sectional dimension. In practice, they are "peripheral" bore holes introduced into the roller body near the outer periphery.
Axially parallel bore holes have been known for quite some time in connection with the heating of rollers. For example, German Utility Model Patent 90 14 117 discloses axially parallel bore holes, that, however, do not form a closed system, but rather are traversed by the flow of a heat-carrier liquid which is heated outside of the roller and transferred by pumping. In the German 40 33 986 A1, bar-shaped, electrical heating elements are arranged in the peripheral bore holes.
In the specific embodiment of the species, the elongated channels must be individually partially filled to a specific level with water and then sealed off in a pressure-tight manner. In addition, after being partially filled with water, the channels must be evacuated, since otherwise air pockets form in the channels and hinder vapor condensation. Because evacuation is necessary, only individually sealing off the channels by welding them closed or by using screw plugs comes into consideration. This requires a considerable expense which may have to be repeated over the service life of the roller. For example, when the roller must be re-engraved, the surface must be heat-treated, making it necessary to empty the elongated channels. It is then necessary to refill the elongated channels with precisely measured volumes of water, and subsequently to evacuate them. The volume of water in each elongated channel should conform, otherwise the condensation heating of the individual channels will not create uniform heating over the periphery of the roller body.